Induction of fetal breathing by metabolic acidemia and its effect on blood flow to the respiratory muscles.

Sustained and vigorous fetal breathing activity was produced in a chronic fetal lamb preparation by infusion into the fetus of either NH4Cl or HCl. Over a 2 to 3 hour period, 20 to 25 mEq/kg were infused. All of the fetuses tolerated blood pH values of 6.7 to 6.8 and survived. The breathing activity began after the completion of the infusion, and consisted of regular 30 to 50 torr inspirations at a rate of 60 to 120 breaths/min. This activity was continuous for as much as 8 hours, and persisted with pauses and decreased amplitude for 24 to 36 hours. During fetal breathing, blood flow to the diaphragm and intercostal muscles increased approximately 12- and sixfold, respectively.